Lepidocrocite Titanate–Graphene Composites for Sodium-Ion Batteries

Abstract

To overcome electronic transport issues of layered titanates in sodium-ion batteries, we have designed and synthesized composites of lepidocrocite titanates with reduced graphene oxide through a solution-based self-assembly approach. The parent lepidocrocite titanate (K 0.8 [Ti 1.73 Li 0.27 ]O 4 ) was exfoliated by a softchemical approach and mechanical shaking. Exfoliated layered titania sheets (LTO) were then combined with reduced graphene oxide (rGO) layers to assemble into composites through flocculation. Countercations (i.e., Mg 2+ ) were used for the selfassembly of negatively charged titania and rGO nanosheets via flocculation. The carbon content in the composites was tuned from 1 to 17% by changing the ratio of titania and rGO sheets in the mixed colloidal suspensions. Electrodes were processed with as-prepared LTO−rGO composites without any carbon additives and tested in sodium half-cell configurations. Mg + -coagulated LTO−rGO composite electrodes deliver higher capacities than electrodes prepared with coagulated titania sheets and 10% acetylene black in sodium half-cells and display good capacity retention after 50 cycles. Electrochemical impedance spectroscopy results indicate lower charge transfer resistance for LTO−14.5%rGO composites than that of coagulated titania sheets with 10% acetylene black. A power law analysis of cells containing the composites indicate a hybrid mechanism consisting of both surface and diffusional processes. A comparison with a similar system, that of dopaminederived LTO-C heterostructures, reveal significant differences. While capacities showed a strong dependence on carbon content for the dopamine-derived materials, this was not true for the LTO−rGO composites. Instead, the highest capacity was obtained for the 14.5% rGO sample, with a lower value obtained for the 17% rGO sample. A greater proportion of the redox processes were surface rather than diffusional in nature for the LTO−rGO composites as well.

Tailoring Stepped Layered Titanates for Sodium-Ion Battery Applications

“…34 Lower capacities were obtained for cells with the graphene-containing heterostructures, with the maximum of about 120 mAh/g achieved for the 8.5% graphene sample. 18 Heterostructures with higher graphene contents than this actually performed worse, probably reflecting the negative effects of graphene restacking on electrochemical performance (Electrodes consisting of pure rGO are electrochemically inert).…”

“…To overcome the low electronic conductivity, we prepared two types of heterostructures, in which carbon is interleaved with the titanate layers. 18,34 Heterostructuring has also been used recently to prepare cathode materials for sodium-ion batteries. 35 The first step, in both cases, was to exfoliate K 0.8 Ti described literature procedures.…”

“…To overcome the low electronic conductivity, we prepared two types of heterostructures, in which carbon is interleaved with the titanate layers. , Heterostructuring has also been used recently to prepare cathode materials for sodium-ion batteries . The first step, in both cases, was to exfoliate K 0.8 Ti 1.73 Li 0.27 O 4 to produce titania nanosheets (Figure a) using previously described literature procedures. , The nanosheets were then combined with either dopamine hydrochloride or reduced graphene oxide (rGO) to form self-assembled heterostructures.…”

“…(b–d) HRTEM images of heterostructured titanate-8.5% rGO. Reprinted with permission from ref . Copyright 2022 American Chemical Society.…”